[m-rev.] for review: bug fix in existential constraints

[Ian MacLarty]

On 14 Apr 2005, at 18:09, Mark Brown wrote:

> Estimated hours taken: 2
> Branches: main, release-0.12
>
> Only allow existential constraints to constrain existentially 
> quantified
> type variables.  Likewise only allow universal constraints to constrain
> universally quantified type variables.
>
> We previously didn't check these conditions, and in fact the reference
> manual allowed otherwise.  However, the implementation did not support
> existential constraints on universally quantified type variables, and
> will not do so for the foreseeable future.  The implementation does 
> support
> universal constraints on existentially quantified variables, however 
> these
> are not useful since no caller will ever be able to satisfy the 
> constraints.
>

Out of curiosity, what does it mean for a universally quantified type 
variable to be existentially constrained?

Ian.

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